This invention relates to an optical tape apparatus for recording, reproducing or re-writing data to and from a tape-like optical recording medium by use of a laser spot, and more particularly to an optical tape apparatus which will be suitable for automatic focusing control and tracking control of an optical VTR (Video Tape Recorder) for recording, reproducing or re-writing data.
A magnetic recording medium has been used generally as a recording medium of data. In comparison with this magnetic recording medium, an optical recording medium using a laser beam for recording and reproducing data, such as an optical disk, has the advantage in that non-contact high density recording can be made.
In the apparatuses which use the tape of the magnetic medium, a magnetic tape that travels at a lower speed is wound helically on a drum that rotates at a higher speed so that signals can be recorded diagonally on the tape and signals such as high frequency video signals can be recorded for a long time. This system is known as a "helical scanning method" and has gained a wide application in VTRs (Video Tape Recorders) for home use in general.
Heretofore, an apparatus has been known which records the data on a tape-like optical recording medium in the same way as the helical scanning method described above to attain high density recording and records large quantities of data such as images for a long time. Such an apparatus is disclosed, for example, in Japanese Patent Laid-Open Nos. 273745/1986, 8339/1987 and 109233/ 1987.
Since high density recording is possible in VTRs using the optical recording medium, such VTRs will be applied to recording and reproduction of video data for a high-definition television in future.
In order to make the VTR structure compact, one or a plurality of heads must be assembled inside a drum and rotated.
In the VTRs using the magnetic recording medium, the drum rotates at from 1,800 to 5,400 rpm and records a high frequency signal. Similarly, the drum must rotate at the same number of revolution as above and record high frequency signals in a high density on the optical recording medium, too. To simplify the structure of the recording optical system, the optical head must be located in the drum. In the optical tape apparatus using the tape-like optical recording medium, the fluctuation of revolution or wobbling of the drum of about 10 .mu.m is expected when the drum is rotated.
In the VTRs using the tape of the magnetic recording medium, the tape travels at a low speed of from 100 to 300 mm/sec on the drum which rotates at 1,800 to 5,400 rpm. For this reason, a nonuniform gap h as shown model-wise in FIG. 24 develops between the drum and the tape depending on the unbalance of the dynamic pressure distribution of an air-film generated by tape tension and the rotation of the drum. In the case of the tape of the magnetic recording medium, recording and reproduction of the data signals can be made stably because the magnetic head for recording and reproducing the data signals projects beyond this gap h and comes in substantial contact with the tape.
In the case of the optical recording medium, however, the signals are recorded and reproduced by focusing or condensing a laser beam to a diameter of about 1 .mu.m by use of a focusing or condenser lens and consequently, high density recording becomes possible. On the other hand, since the size of the optical spot to be condensed on the optical recording medium is small, recording and reproduction of the data signals will not be stable if any oscillation occurs in the direction of the focus of the optical head due to fluctuation or wobbling of rotation of the drum during the travel of the tape. If the wavelength of the laser beam is .lambda. and the numerical aperture of the condenser or objective lens is represented by NA, the depth of field or focal depth of the condensed laser spot is approximately to .lambda./(NA).sup.2. Assuming that a semiconductor laser beam having a wavelength of 830 nm is used and NA of the condenser or objective lens is 0.5, then the focal depth is about .+-.1.7 .mu.m. Therefore, unless the tape oscillation during the travel of the tape is kept below this value, the size of the laser spot will fluctuate and recording and reproduction of the signals cannot be made stably.
In the case of the optical recording medium, if the non-uniform gap such as shown in FIG. 24 exists, the focal position deviates with the rotation of the drum and this results directly in the error of focus. Furthermore, the tape will oscillate very delicately due to the pressure fluctuation inside this gap and to the fluctuation of the tape travel.
Accordingly, the focus must be adjusted on the optical tape by an automatic focusing control mechanism. This automatic focusing control is referred to as "AF", and drives the condenser lens in the focal direction by use of a voice coil motor using the same principle as that of speakers so as to absorb the tape fluctuation.
In the optical tape apparatus, the drum has a diameter of about 60 mm and a number of revolution of from 1,800 to 5,400 rpm, for example. Since the signals are recorded on the optical tape, the condenser lens must be disposed near the outer periphery of the drum. Assuming that the lens is positioned at the outermost periphery of the drum which rotates at 5,400 rpm, the centrifugal force is about 1,000 G.
In other words, if the weight of the condenser lens is 5 g and the distance from the center of the drum to the condenser lens is 30 mm, the centrifugal force of about 5 kg acts on the condenser lens. If the position of the condenser lens is offset by this centrifugal force, unnecessary power will be required in the driving circuit of the automatic focusing servo system in order to correct this offset or the error of automatic focusing will remain.
In the optical recording medium, recording and reproduction of signals are made by directly condensing the laser beam to a diameter of about 1 .mu.m by the lens and high density recording can be made accordingly. However, since the dimension of the data recorded on the optical recording medium is too small, tracking of the data track as the target must be made accurately at the time of recording and reproduction. Particularly when the data recorded by the helical system is recorded and reproduced, the data track becomes discontinuous whenever the drum rotates once. Therefore, accurate tracking becomes a critical problem though it has not been the critical problem in the case of the recording and reproduction head using an electromagnet for the magnetic recording medium, where a width of as large as about 20 .mu.m is secured in the tracking direction.
In the case of the tape-like optical recording medium, in particular, the optical recording medium travels while being wound helically on the drum and for this reason, there is a great possibility that the fluctuation of the tape travelling speed occurs, and accurate tracking of the data track as the target becomes difficult at the time of recording and reproduction of the signals.
In comparison with a rotary disk such as an optical disk, the speed fluction is more likely to occur in the tape-like recording medium when the tape travels, and it is more difficult to transfer the guide track from a master the same as in the optical disk.
The prior art technique described already does not much consider the guide track for recording and reproducing accurately the data to and from the target position, and involves the problem in the tracking at the time of recording and reproduction of the data.
As described above, the prior art technique does not much consider the prevention of the fluctuation of the tape travel and the adverse influences of the centrifugal force resulting from the high speed rotation of the optical head on the actuator. It further involves the problem in automatic focusing control and tracking control.
Various actuators have so far been proposed in the optical disks. However, at many times the optical head is substantially at rest relative to the disk at the time of recording and reproduction of the signals, and a voice coil motor using a parallel leaf spring, a voice coil motor using a rotary type slide bearing, a galvanomirror, and the like, have mostly been used for the actuator. In the case of the optical tape, the optical head component(s) positioned on the outer periphery of the drum must have high rigidity because a large centrifugal force acts on them. Particularly, the actuator which makes automatic focusing control and tracking control by adjusting the position of the condenser lens must have a structure providing a sufficient stroke and having high rigidity.